A form of pressure treatment, typically for patients with obstructive sleep apnea (OSA), is continuous positive airway pressure (CPAP) applied by a blower (compressor) via a connecting hose and mask. The positive pressure may be used to prevent collapse of the patient's airway during inspiration, thus preventing recurrent apnoeas or hypopnoeas and their sequelae. Such a respiratory treatment apparatus can function to generate a supply of clean breathable gas (usually air, with or without supplemental oxygen) at the therapeutic pressure or pressures that may change to treat different events but may remain approximately constant across a given cycle of the patient respiration cycle (i.e., inspiration and expiration) or may be reduced for comfort during each expiration (e.g., bi-level CPAP).
Respiratory treatment apparatus can typically include a flow generator, an air filter, a mask or cannula, an air delivery conduit connecting the flow generator to the mask, various sensors and a microprocessor-based controller. The flow generator may include a servo-controlled motor and an impeller. The flow generator may also include a valve capable of discharging air to atmosphere as a means for altering the pressure delivered to the patient as an alternative to motor speed control. The sensors measure, amongst other things, motor speed, gas volumetric flow rate and outlet pressure, such as with a pressure transducer, flow sensor or the like. The apparatus may optionally include a humidifier and/or heater elements in the path of the air delivery circuit. The controller may include data storage capacity with or without integrated data retrieval/transfer and display functions.
In addition to apnoeas or hypopnoeas, patients on pressure treatment therapy, such as CPAP therapy, might also experience hypoventilation. Hypoventilation may be considered an occurrence of a persistently low ventilation as opposed to a periodic pause or apnoea. Such incidents of hypoventilation may not be recorded or recognized by the patient or by a clinician or physician managing the patient.
In a case of the patient receiving CPAP therapy, hypoventilation may present for a number of reasons:
(a) The patient may have a primary diagnosis of obesity hypoventilation syndrome (OHS) and CPAP is being trialed as a therapy. Many patients recover over months with such treatment. Although overnight oxygen blood saturation (SpO2) recording is the standard way of identifying efficacy, this is potentially costly and cumbersome.
(b) The patient may develop hypoventilation over time due to an underlying disease process (e.g., chronic obstructive pulmonary disease (COPD) progression or OHS progression from obstructive sleep apnea (OSA)).
(c) The patient may have had a limited diagnostic test where the hypoventilation was missed Dr, for example, they did not enter the sleep state where hypoventilation presents (e.g., supine sleep).
It may be desirable to develop methods for evaluating or accessing patient ventilation, which may also be implemented in apparatus for assessment of ventilation or apparatus for generating a respiratory pressure treatment.